1. Field of the Invention
This invention relates techniques for utilizing solar energy and, more particularly, to energy efficient barriers which are selectively oriented to pass or block direct solar energy.
2. Prior Art
Conventional skylights and window blinds in buildings often provide undesirable paths for solar energy gains and heat losses. For example, in winter it is desirable to maximize the amount of solar energy entering a building, while in summer, it is desirable to minimize the amount of solar energy entering a building. A problem is that conventional skylights and window blinds provide solar energy gains and losses which are the opposite of what is desired during a particular season.
A number of techniques have been developed for optimizing the energy gains and losses through skylights and windows. One technique for returning solar energy uses a large reflective prismatic panel for window or skylight glazing. A very thin film with a thickness of 20 thousandths of an inch is solvent bonded to an acrylic sheet to form a panel. The panel has a front surface into which sunlight enters and the film provides a series of prisms which are internally formed on the back wall of the panel. Light enters the front surface of the panel and passes through the panel to the back wall. The light is then reflected back from the prisms internally formed in the back wall of the plate using total internal reflection (TIR). The reflected light passes again through the panel and out of the front surface of the panel. The panel is fixed in its orientation to the direct rays of the sun and is not adjustable or selective in its operation. Its operation does not easily account for seasonal differences, where it may be desirable to either transmit or block solar energy from passing therethrough. One problem with this technique is that light is trapped and returned only within a narrow acceptance angle. Light falling outside of the acceptance angle passes through the film. This technique has an acceptance angle, for example, of plus or minus 10 degrees. It is desirable that the acceptance angle be on the order of plus or minus 30 degrees.
Venetian blinds for windows and skylights have a number of horizontal slats which are simultaneously adjustable at different angles to vary the amount of solar energy passed through. A paper published in Solar Energy Vol. 44, No. 3, pp. 157-160, 1990 by the Pergamon Press entitled "Experimental Study of Full-Size Automated Venetian Blind Windows" discloses an automated venetian blind system with horizontally pivoted louvers which are hermetically sealed between two glass panes. The louvers are fixed at both ends to the sides of a window unit and can be rotated around a horizontal axis by a mechanical system. A problem with this concept is that the blinds heat up and then transfer heat to the inside of the building by radiation and conduction. Three major heat-exchange mechanisms are present in this arrangement: radiative; convective exchange both inside and outside of the cavity defined by the glass panes; and conductive exchange in the wall of the building where the system is installed.
Consequently, the need has arisen for techniques to optimize the amount of solar energy either passed or reflected by a skylight or window for the different seasons.